【作者】 陈忠华；

【导师】 祖方遒；

【作者基本信息】 合肥工业大学 ， 材料加工工程， 2010， 硕士

【摘要】 过共晶铝硅合金因具有耐磨、体积稳定等优异特性,在汽车发动机等领域取代传统材料已成为发达国家一种趋势。而国内因其初生硅组织控制难度等原因未能广泛推广。另一方面,如何通过操控熔体状态、变质等措施,来控制凝固组织,涉及凝固基础及凝固技术的发展。本文首先运用电阻法探索了Al-18%Si熔体状态与温度关系,然后基于此信息分别以熔体过热处理及高-低温混溶处理进行了两方面凝固实验,探索了熔体状态对过共晶铝硅合金中初生及共晶硅的作用与规律。在高-低温混溶处理中,论文还创新性地探索了低温熔体处于半固态温度范围的作用效果。论文工作还探索了P-Re-B中间合金对Al-18%Si合金的复合变质作用。全文主要研究内容及结论如下:1、升温过程Al-18%Si熔体电阻率温度行为(ρ-T曲线)揭示,液相线以上从800℃到950℃的温度区间内,ρ-T曲线出现一个峰高较低而宽的预峰。而从950℃到1050℃的温度区间内,ρ-T曲线上出现另一窄而明显的温度峰,此后熔体电阻率进入新的线性变化阶段。分析认为,第一峰发生预转变,第二峰处发生了不可逆液液结构转变,结果成为一种更为均匀的结构状态。2、通过对Al-18%Si合金进行熔体过热处理及凝固实验发现,950℃保温的熔体,快冷至760℃后凝固得到的初生硅,其晶粒大小与760℃处保温的熔体凝固后的差别不大;而1150℃处保温的熔体,快冷至760℃后凝固得到的初生硅已得到明显细化。冷却曲线反映的过冷度差别与凝固组织的规律基本一致。这一结果也验证了上述“不可逆液液转变”的推论。3、高-低温混溶处理以1150℃为高温熔体温度,其低温熔体的温度分别选取处于半固态温度范围的620℃、640℃及高于液相线的700℃、760℃,并改变高、低温熔体比例。凝固实验结果表明,低温熔体温度为640℃混溶所获得的试样其初生硅最细小,且随高温熔体比例的增大,初生硅晶粒随之细化。从机理上看,640℃最利于半固态范围内已有块状分枝的熔断和增殖;而高温熔体比例的提高,越发促进硅晶体熔断和增殖,增加形核核心。4、在选定低温熔体温度为640℃后,改变高温熔体的热历史,处理方式分别为1150℃、1150℃-950℃、950℃,发现经历过不可逆转变的高温1150℃熔体,比未经历转变的熔体对初生硅有着更好的细化作用。5、运用P-Re-B中间合金对Al-18%Si合金进行复合变质,对初生硅和共晶硅均有明显改善作用。在最佳的P-Re-B复合变质工艺下( AlP0.3%+ AlRE2%+AlTiB1.2%),初晶平均直径由未变质的33.7μm细化到9.00μm,形态由多角粗大板块变为细小颗粒,棱角钝化,分布更加均匀;并使共晶硅由粗针状变为纤维状。本文研究结果显示,基于Al-18%Si熔体结构状态对温度关系的信息来实施熔体热处理,可避免通常的盲目性取得更好的效果。更多还原

【Abstract】 In developed countries, hypereutectic Al-Si alloy has a tendency to take place traditional materials because it is wear-resistant with volume stability. But in China, this material has not been popularized due to the difficulty to control the primary silicon microstructure. On the other hand, how to manipulate the solidification structure by controlling melt state, modification and other factors requires the improvement of fundamental solidification and solidification technology. This paper firstly explores the relationship between the melt state of Al-18%Si and its temperature by means of the resistivity method. Based on this obtained information, two groups of solidification experiments (overheating treatment of melt in one group and high-low temperature melt mixed treatment in another group) are carried out to explore the influences of the melt state on primary silicon and eutectic silicon respectively in the hypereutectic Al-Si alloy. This paper also creatively investigated into low-temperature melt which is in the semi-solid temperature zone and high-low temperature melt mixed treatment. What’s more, the complex modification function of P-Re-B master alloy to Al-18%Si alloy is discussed here. On the whole, research contents and conclusions of this paper are as follows:1. The resistivity-temperature behavior of Al-18%Si melt (ρ-T curve) in the temperature-rising period reveals that between 800℃and 950℃above the liquidus, there is a peak with low and wide peak height in theρ-T curve. But between 950℃and 1050℃, there is a narrow but obvious temperature peak in theρ-T curve. Afterwards the resistivity of the melt enters into a new linear change phase. It is indicated that pre-transformation occurs in the first peak and irreversible liquid-liquid structural change occurs in the second peak with a result of a more even structural behavior.2. By dealing with Al-18%Si alloy with overheating treatment and solidification experiments, it is found the grain size of primary silicon, obtained from melt insulated at 950℃and rapidly cooled to 760℃, has no evident difference compared with the one solidified from insulated melt at 760℃. However, primary silicon which is obtained from melt insulated at 1150℃and rapidly cooled to 760℃, is obviously refined. The undercooling difference reflected by the cooling curve is basically in accordance with the law of solidification microstructure. 3. In high-low temperature melt mixed treatment; the high melt temperature is 1150℃while the low melt temperatures are respectively 620℃and 640℃among the semi-solid range as well as 700℃and 760℃above the liquidus line. And the proportion of melts at high temperature to the ones at low temperature is changed. The results show that primary silicon in the sample obtained by mixing melts at 1150℃and 640℃is the finest. In addition, the bigger the proportion of high-temperature melt is, the finer the crystal grain of primary silicon is. Seen from the mechanism, 640℃is the most favorable to fusing and proliferation in silicon dendritic crystal among the semi-solid range; and the rise of the proportion of high-temperature melt promotes fusing and proliferation of silicon crystal and increases the nucleus.4. After 640℃is decided as the low melt temperature, change the thermal history of high-temperature melts. The temperatures are 1150℃, 1150℃-950℃and 950℃respectively. It is found that high-temperature melts at 1150℃undergone irreversible transformation has better refining effect to primary silicon than the melts which have not undergone such transformation.5. Conduct compound modification to Al-18%Si by using P-Re-B master alloy. It has obvious positive influence on both primary silicon and eutectic silicon. Under the optimal P-Re-B compound modification technique (AlP0.3%+ AlRE2%+AlTiB1.2%), the average diameter of primary crystals is refined to 9.00μm from previous 33.7μm; their shapes change from coarser and brittle to more-edges-distributed fine particles with passivated edges; the eutectic silicon is also changed from acicular to fibrillar.To sum up, the results of this paper show it can avoid blindness and achieve better effects by carrying out heat treatment to melts based on the relationship between Al-18%Si melt structural state and temperature.更多还原